1. Field Of The Invention
The invention relates to gas absorption cells and, more particularly, to a compact multiple pass open path gas absorption measurements in a compact space using an extended light source, such as thermal, hot filament, gas discharge, etc.
2. Description of the Related Art
Optical sampling cells are commonly utilized in IR spectrophotometers to measure the concentration of gas within the sampling cells. Specifically, multiple pass optical sampling cells are widely used in IR spectrophotometers to measure low concentration gases or gases having very faint absorption bands. Both single and multiple pass sampling cells have a number of limitations which diminish their effectiveness as analytical tools.
Specifically, single pass sampling cells conventionally have an optical path length which is essentially the same as the physical path length. Single pass sampling cells are designed such that a light bundle enters the cell at one end and exits at the other end without making any reflections. As a result, the cell length required for measuring gas absorption may be impractical when a gas having a low absorption coefficient is measured. Additionally, similar problems arise when low densities or concentrations of gas are present. It should be noted that "light" or "light bundle" as referred to herein means any form of electromagnetic radiation used to measure the absorption of predetermined gases.
When single pass sampling cells are used with extended light sources, the light source may strike the interior walls of the sampling cell at shallow angles and be reflected many times. Clearly, this reflected light does not follow the simple intended path. Instead, multiple reflections cause the reflected light to take a longer path through the sampling cell. When absorbance in a sampling cell is interpreted by applying the Beer-Lambert relation, which assumes a single well-defined length, such reflections can cause significant errors. This effect is often exaggerated for longer sampling cells with narrow diameters.
For applications involving the measurement of reactive trace gases at low pressure, it is important to minimize collisions with the sampling cell walls and to move the gas through the cell rapidly so that the composition of the sample does not change during the measurement. Both conditions are difficult to achieve with a single pass sampling cell small enough for use with practical instruments.
With regard to multiple pass sampling cells, the White cell and the Herriott cell are exemplary of commonly employed sampling cells. The White cell utilizes a cell having a pair of mirrors opposite the end of the cell through which light enters. Each of the mirrors must equal or exceed the size of the light bundle's diameter after it traverses the sampling cell once. It should be noted that the diameter of the light bundle, as referred to in the body of this application, is meant to indicate the diameter of the light bundle taken in a plane perpendicular to the longitudinal axis of the light bundle. Each of the paired mirrors must be larger than the expanded light bundle, so that end of the sampling cell must be more than twice the light bundle diameter in at least one direction.
The Herriott cell includes two spherical mirrors of equal radii on opposite ends of the sampling cell. The centers of the light bundle makes repeated reflections from the mirrors, striking them at locations away from the centers of the mirrors. As a result, the mirrors must have diameters considerably larger than the maximum diameter of the light bundle. Thus, the Herriott cell is best suited for use with small, well disciplined light bundles.
Multiple pass sampling cells provide long optical path length in a relatively small volume. They are, however, very sensitive to misalignment. A small change in mirror separation can change the number of reflections and produce large changes in the path length. In addition, when prior multiple pass cells are employed with an extended source, it is possible for light to follow a path significantly different from the intended path. This introduces inaccuracies into measurements that generally have little room for error.
Additionally, where a sampling cell is used for accurately measuring gas concentration by applying the Beer-Lambert relation, it is necessary to accurately determine the length of the light path within the sampling cell. The problems discussed above can adversely affect the accuracy of measurements, are difficult to quantify, and consequently make it very difficult to apply the principles of the Beer-Lambert relation.
U.S. Pat. No. 4,676,652 to Chernin et al. discloses a multiple pass optical system using curved transfer mirrors in which light strikes one mirror of a pair at a more glancing angle than the other. Other U.S. patents disclosing a variety of optical systems include U.S. Pat. Nos. 3,726,598 to Gilby; 4,035,963 to Gilby; 4,209,232 to Chernin; 4,626,078 to Chernin et al.; 5,009,493 to Koch et al.; and 5,125,742 to Wilks.
A continuing need exists for a multiple pass sampling cell capable of providing convenient, reliable and efficient measurements.